Electron discharge apparatus



June 1, 1943. J. H. FREMLIN ELECTRON DISCHARGE APPARATUS Filed Nov. 27, 1940 2 Sheets-Sheet 1 Fig. 2d.

//VVENTOR Xfljzmbv er I I ATTOR/VE I June 1943- J. H. FREMLIN 2,320,369

ELECTRON DISCHARGE APPARATUS I I Filed Nov. 27, 1940 2 Sheets-Sheet 2 Patented June 1, 1943 2.32am v Emerson mscnsnen arrsaarus John Beaver Fremiin, Eondon,

England, assignmto International Standard Electric Corporation,

New York, N. Y.

Application November In G 11 Claims.

This invention relates to electron discharge apparatus for operation at extremely high frequencies.

In one aspect of the invention electron discharge apparatus i'or operation at extremely high frequencies comprises a substantially closed hollow re-entrant shell serving as a resonator and means for directing electrons through apertures in the outer walls and the re-entrant walls of the resonator so that the electrons in entering the resonator are controlled in velocity and in leaving the resonator collectively yield more energy to the resonator than they take from it.

In another aspect the invention comprises a substantially closed hollow resonator consisting 01 an outer shell and an inner portion extending between opposite sides of the shell and means for directing electrons to pass across the outer shelland through the inner portion so that electrons in passing from the outer shell to the inner portion of the resonator are controlled in velocity and in passing from the inner portion to the outer shell collectively yield more energy to the resonator than they take from it.

In another aspect electron discharge apparatus of the electron-velocity-modulated type comprises as a resonator a short length of concentric-conductor transmission line with closed ends and having aligned diametric slots or apertures and electrodes for passing electrons through said slots or apertures to flow completely through the concentric line along a diameter thereof.

The resonator may alternatively be in the form of a short cylinder with annular disc ends and with a cylinder of relatively small diameter mounted coaxially within it, for example, by means of a metal spider or disc extending over a central diametric plane which is a voltage nodal point, electrons being directed anally through the resonator to cross successively gaps between the annular discs and the inner cylinder.

The invention including subsidiary features and preferred embodiments will be described with reference to the accompanying drawings in which:

Fig. 1 is a diagrammatic-view oi a known kind of velocity-modulated tube.

Fig. 2a is a perspective view of a resonator for use in apparatus incorporating the invention.

Fig. 2b is a horizontal section through apparatus incorporating the invention and comprising a resonator similar in essentials to that of Fig. 2a.

Figs. 2c and 2d are vertical sections at right angles of a resonator such asthat of Fig. 2a.

27, 1940, Serial No. 367,401 seat Britain December 22, 1939 The chief cause of this is the fact that electrons take a finite time to transverse the space between difierent electrodes. In addition, it is dimcult adequately to reduce the internal capacities and inductances. The transit time of the electrons can be reduced either by increasing the voltages on the tube electrodes or by decreasing the linear dimensions of the electrodes, but practical considerations limit the extent to which transit time can be reduced. In consequence, if we wish to obtain very short wavelengths we must find some method by which the eifect of the electron transit time is made not harmful. As it is impossible to eliminate transit time, there are two things which we can do; either we can render the transit time harmless, or we can actually use the transit time to help us to maintain the desired oscillations. Y

The magnetron and B-arkhausen tubes are both well known. These use the transit time of the electrons to maintain the oscillations which are produced in them. There is also the principle of velocity modulation of electrons proposed in British Patent No. 431,447 and in other publications by O. Heil. The principle of velocity modulation does not actually use directly the transit time of the electrons, but makes the transit time harmless. In one type of electron discharge device utilising velocity modulation and known as the Klystron, two enclosed hollow resonators Al and Bi, Fig. 1, tuned accurately to the same frequency are placed in such a way that an electron beam K traverses both of them. The two resonators or resonant circuits are coupled together, if the tube is to be used as an oscillator, and the process by which they work is as follows: Supposing that a small oscillation is going on in the first resonator Al, then an electron beam traversing Ai will at times be accelerated by the electro-magnetic oscillation, and at times it will be retarded. We have, therefore, in the beam leaving the resonator Al, electrons of different velocity according to the phase in which they traverse Al. As the beam travels along the gap between Al and BI, usually described as a drift tube, electrons which have lost energy in the beam will no longer be uniform but will gather themselves up in a series or bunches which travel on with approximately the mean velocity or the beam. The resonator BI is placed so that this ounching process hasgone on to a considerable extent when the electrons arrive at Bl. BI is traversed by these series of bunches at regular intervals. Now it Bl itself is oscillating, and if it is in such a phase that the electro-magnetic field due to the oscillation opposes the electron beam as each bunch comes through, and accelerates the electron beam during the subsequent rareiaction, the electron beam will on the average lose more energy to the oscillation than it gains from the oscillation, i. e., the kinetic energy of the beam is being transformed in part into energy of electro-magnetio oscillation in Bl It is sufilcient for quite a weak coupling, 1. e., for the system to build up electro-magnetic oscillations. The frequency 01' these oscillations is of course determined absolutely by the characteristics of the resonators Al and BI. These resonators are usually constructed of copper sheet, symmetrically, and are re-entrant as shown so that the time taken by the beam to cross the gap is not long compared with the period of the oscillation.

A number of difierent shapes for the resonators has been proposed, the main advantages being purely mechanical. Such resonators have extremely low losses, owing to the low value of capacity and the very large area of copper included in them, and consequently they may have Q of the order of 20,000 to 50,000. This means of course that the resonators must be very accurately tuned to each other in order that oscillations may be successfully maintained.

It is now shown to be possible to set up a system in which the principle of velocity modulation is used, but in which a single resonator is employed. What is required to do this is, a single closed hollow resonator including a cylindrical reentrant shell provided with a dlametrical passage therethrough consisting of slots in alignment with each other and having fins or flanges extending from the edges of the slots, so that the electrons can pass across one part of or gap in the circuit in which their velocities are controlled, and they can then, after a considerable space corresponding to the drift tube in Fig. 1, pass across another part of the circuit, in which their energy may be given up.

As shown in Fig. 2a the single resonator may comprise a short length of concentric line comprising hollow conductive cylinders A and B closed at both ends with copper discs. It is clear that this could oscillate as a. half-wavelength section of a long concentric line in which there were standing waves, the two ends of the section being potential nodes and the centre of the section being a potential anti-node. straight through the system along a diameter at the anti-node in order to obtain the results mentioned as desirable above. In the preferred structure the slots which have been out both in the outer cylinder of the concentric line and in the centre cylinder have metal fins or flanges C, D, E put on to them for two reasons. One is to prevent the otherwise extensive leakage of the electro-magnetic field out of the cylinder, and

the other is to decrease the radial gap between the two cylinders across which the electrons have to pass. Now suppose that an electron beam is fired through the resonator. A suitable arrange- A slot is cut' ment is shown in Fig. 2b where H is a heated cathode, G a slotted plate or strip serving as an electron beam defining and modulating grid and F a collector anode. A separate 'collector anode F is not essential; it may be integral with the resonator. The aperture at the outer edge of the fins E may be closed to form a collecting surface. The electrons are considered to have left the resonator as soon as they have entered the space between fins E since they have no further effect unless they emerge again with substantial velocity. An axial magnetic field or an electron gun including the disc G is required to drive the electrons from cathode H through the resonator A, B to collector anode F. It is clear that the electrons passing through the resonator, as they cross the first gap, will come into the field it the system is oscillating and will be velocity modulated, as has been described above. During the rather short fiight between the fins on the centre conductor they will sort themselves out in bunches, which then will give up energy to the electro-magnetic oscillation as they cross the second gap.

By using such a system we gain the big advantage that oscillations can be obtained without the necessity for tuning two high Q circuits to exactly the same frequency. On the other hand, the overall theoretical efiiciency of the valve is less than in systems such as in Fig. 1.

The theoretical limit for a velocity modulated power tube of the Klystron type is 58%. The theoretical limit for a single circuit tube of the particular type described is about 35%. This means that for a good power output in the latter, a greater amount of power must be dissipated on the final collecting electrode. It is of course true, however, to say that in neither case will it be possible to reach the theoretical limits of efficiency. The Klystron type of tube has two circuits to be tuned, and a considerably longer path over which the electrons have to travel between the two circuits. In consequence of this it will be more difilcult to reach the theoretical efiiciency than in the case of the single circuit tube, where the chances of deviation of electrons from their proper paths are considerably less. In order that such a single circuit tube as has been described should have an efiiciency approaching 35%, it is necessary that its dimensions should be accurately calculated. In the specific structure described, the first gap through which the electrons travel, the control gap, should be so dimensioned that, at the voltage at which it is desired the valve should operate, the electrons will travel across the first gap in 194 out of the 360 for a full cycle. The working gap, on the other hand, however, which they pass later, after passing the centre conductor, should be as narrow as is convenient. Any increase in width of this gap leads to loss of efilciency, but it cannot be made too narrow without increasing the capacity between the centre conductor and the outside tube so greatly as to increase the wavelength of the oscillation to a serious extent. Suitable shapes of slots and fins are indicated by way of example in Figs. 2c and 2d.

The use of a magnetic field to ensure that the electrons follow the path through the electrode structure readily permits an electron beam of high intensity tobe achieved, but as an alternative an electron gun may be used.

A discharge tube with a single resonator as described can relatively easily be made to work on a number of different frequencies. It is possible by the use of metal tubing of flexible bellows type or other similar means to increase the length of the tube to a considerable extent, using a micrometer adjustment for operation over a range of frequencies as great as 25% of the mean frequency. In this way it has a considerable advantage over tubes of the Klystron type employing separate resonators for controlling the :beam and for obtaining energy from the beam in view of the difilculty in matching the resonators at each frequency.

The output power of the discharge tubes described depends among other things uponthe electron current intensity through the resonator.

It is possible to divide the slots up by par titions or fins as shown in Fig. 3 instead of having a simple cross section as in this way two, three or more parallel electron beams are formed in the gaps and the total electron current can be increased by increasing the cross-section of the initial beam. At the same time it is worth noticing that the capacity is not increased by the same factor as is the current (assuming the main capacity to lie between the this on the centre conductor and on the outside tube).

Where the magnetic field is used, it is impossible to use electron beams directed at an angle to one another. If an electro-static system is used, however, it is possible to use a system such as that shown in Fig. 4, in which three or more beams of electrons are fired in different directions round the outer tube, and arrive at a corresponding number of anodes on the side away from the point of entrance. This system has a considerable advantage from a mechanical point of view, and from the point of view that the anodes are more easily kept cool than in the case envisaged in Fig. 3.

Another method of focussing the electron beam to pass through he necessarily rather narrow slots in the conductors is to use an inhomogenous magnetic field. When an electron beam is allowed to travel along a magnetic field, the individual electron paths follow closely the magnetic lines of force, and actually consist of closely wound helices round the magnetic lines of force.

If a magnetic field be set up having widely spaced lines of force on the cathode H of a. valve, and closely spaced lines of force in the resonator A as shown in Fig. 5, it is possible to focus electrons emitted over a relatively wide area into a path of considerably smaller cross-sectional area. In Fig. 5 the chain lines indicate both the necessary directions of the lines of force and the resultin paths of the electrons, the relation of the parts of the resonator A and the operation thereof being similar to that described in connection with Figs. 2a., 2b.

Various modifications particularly in the shape of the resonator within the scope of the appended claims will be readily appreciated.

What is claimed is:

1. Electron discharge apparatus comprising a substantially closed hollow resonator including an outer portion and a conductive reentrant portion extending between opposed surfaces of said resonator, said resonator having a set of apertures extending along a line passing through said resonator and through said reentrant portion, and electron gun means external of said resonator and aligned with said 'set of apertures, whereby electrons entering the resonator are controlled in velocity and in leaving the resonator collectively yield more energy to the resonator than they take from it.

2. Electron discharge apparatus including a pair of coaxial hollow conducting members, a short-circuiting connection between said members at each end of said members, said members having a set of substantially diametrically aligned apertures, generally radially extending flange portions on said members at said apertures, and electron gun means aligned, with and directed at a set of said apertures.

3. Electron discharge apparatus including a pair of coaxial hollow conducting members, a short-circuiting connection between said members at each end of said members, said members having a plurality of sets of substantially diametrically aligned apertures, generally radially extending flange portions on said members at said apertures, and electron gun means aligned with and directed at each 01 said sets of apertures. 4. Electron discharge apparatus according to claim 3, in which said sets oi apertures intersect the axis of said hollow members at substantially the same point. p

5. Electron discharge apparatus according to claim 6, in which said apertures are disposed at substantially the axial midpoints of said cylindrical chambers. Y

6. Electron discharge apparatus for operation at very high frequencies including a resonator having substantially closed hollow re-entrant walls including a generally cylindrical outer shell and a generally cylindrical shell or smaller diameter than said outer shell and located substantially coaxially within said outer shell, said shells having a set of diametrically aligned apertures providing an electron passage extending completely through said cylinders, and means for ,causing a beam of electrons to pass from outside said outer shell through said successive apertures, whereby electrons entering the resonator are controlled in velocity and in leaving the resonator collectively yield more energy to the resonator than they take from it.

7. Electron discharge apparatus for operation at very high frequencies, including a substantially closed, hollow resonator having an outer shell and a shell within said outer shell, said inner shell extending between and having connections with opposite sides of said outer shell, said inner shell being spaced from said outer shell at parts thereof between said connections with said outer shell, said shells having a set of aligned apertures providing a path extending completely through both of said shells, means aligned with said apertures for drawing electrons completely through said apertures, whereby electrons in passing from the outer shell to the inner shell of the resonator are controlled in velocity and in passing from the inner shell to the outer shell collectively yield more energy to the resonator than they take from it, and means for collecting said electrons.

8. Electron discharge apparatus of the electron velocity-modulated type. including asa resonator a short length of concentric-conductor transmission line, a short circuiting connection at each end of the line, said line having diametrically aligned slots having flanges extending therefrom, and electron gun means aligned with said slots for 1 passing electrons through said slots from one end of the diameter of said resonator to the opposite end of said diameter.

9. Electron discharge apparatus according to claim 1, in which flanges extend in a generally radial direction from the resonator at said apertures and provide gaps of predetermined length along the path traversed by the electrons passing through said apertures.

10. Electron discharge apparatus according to tween said resonator portions in the path of elecciaim l, in which flanges extend in a generally trons passing successively through said apertures. radial direction from the resonator at said aper- 11. Electrical discharge apparatu according to tures, the resulting first sap between the outer claim 8, in which said times at the Opp ite portion of said resonator and the re-entrant por- 5 sides of each said slot, are parallel with each tion of the resonator, traversed by electrons from other. said gun, being longer than the second gap be- JOHN HEAVER mnmm. 

